Microstrip Hairpin 3.5GHz Filter
I recently designed a hairpin 3.5GHz microstrip filter for a university assignment. I was lucky to be able to take the subject as an elective, which was brilliant because it is the only exposure I get to practical high frequency design.
Given a set of filter characteristics, such as centre frequency and bandwidth, I designed the hair pin filter by calculating line geometries in MATLAB and online tools, used CST to simulate a single resonator and calculate Q-factors, resonance and k values and then simulated the entire 5 resonator circuit. CST in an incredible power tool for this sort of design, which has been handy for a PCB antenna I needed to design for another project, but because all designs that use it are proprietary it’s stupidly difficult to get help using it, resulting in what has been dubbed the “CST experience” of trial and error.
After much wrestling with CST, I was happy with the simulated s-parameters and submitted the design to the lecturer, where he etched the design on a Rogers PCB. Here it is:
Testing it on the VNA showed that the centre frequency is 3.449GHz (1.47% off the required 3.5GHz), bandwidth is 275.3MHz (1.66% off the required 300MHz) and an average insertion loss of 2.786dB (no requirement on this). S-parameters:
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